Applying hydrodynamic conditions, which certify a negligible influence of convective diffusion, the time-dependent uptake of thionin in lymphocytes, monkey kidney cells, and their separated nuclei was measured spectroscopically. Using fixed cell material the dye transport inside the cell is not hindered due to plasma membrane and cytoplasm. The staining rate depends on the dye concentration, the pretreatment of the cell, and on the electrolyte concentration of the dye solution. The mechanism of dye migration inside the cell is in accordance with a porous matrix model. The diffusion process takes place inside the pores and channels filled with liquid and is modified by adsorption of dye molecules on the walls of the pores. A dynamic reversible equilibrium exists between migrating dye molecules and the binding sites on the pore walls described by the Freundlich adsorption isotherm. The proposed model explains the observed order of reaction of the staining kinetics.